Britain swelters through hottest June day in 41 years as heatwave scorches from London to Siberia

Europe has sizzled in a continent-wide heatwave overnight, with London seeing Britain's hottest June day since 1976 and Portugal continued battling deadly forest fires.

More than 1,000 firefighters were still fighting to control the flames that broke out in central Portugal at the weekend, killing 64 people.

Cooler weather was aiding their efforts, but thermometers were still hovering around 35C -- a level matched across oven-like swathes of Europe, including Italy, Austria, the Netherlands and even alpine Switzerland.

Sunbathers lie on the grass in Hyde Park, London. (AAP) ()People soak up the sun on a beach in Aldeburgh, Suffolk, in the United Kingdom. (AAP) ()

Two forest fires have also broken out since Sunday on Croatia's southern Adriatic coast, prompting the authorities to evacuate 800 tourists, though the blazes have now been brought under control.

"We were scared, it's true," a Swedish tourist who gave her name only as Karolina told local media. "From the hotel room window it seemed like the fire was at our doors."

As the northern hemisphere marked the summer solstice, firefighters were on alert in Austria, where revellers were due to celebrate the longest day of the year by lighting bonfires for the festival of Saint John.

In Italy, forecasters say the current heatwave could turn out to be the most intense in 15 years, with temperatures around eight degrees above the seasonal average -- 39C in Milan and up to 30C in the Alps at an altitude of 1000m.

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A study released Monday warned that deadly heatwaves will become more and more common around the world even if the rise in temperatures is capped at 2C as targeted by the Paris Agreement to combat climate change.

"Even if we outperform the Paris targets, the population exposed to deadly heat will be about 50 percent by 2100," Camilo Mora, lead author of the study published in the journal Nature Climate Change, told AFP.

Britain saw its first five-day stretch of temperatures over 30C in June since 1995 -- and the thermometer hit 33.9C in west London, a UK record for the month since 1976.

In Guildford, southwest of the capital, a road surface melted on Tuesday, with motorists likening it to a bar of chocolate left out in the hot sun.

Fans spray water on people at a bar in Madrid. (AFP) ()People refresh themselves in the Trocadero fountain in Paris. (AFP) ()Tourists splash water on themselves at the Trevi Fountain in Rome. (AFP) ()

At the prestigious Royal Ascot horse races, organisers said they were considering relaxing the strict dress code for the first time.

And a thunderstorm was looming over the Glastonbury Music Festival, threatening mud just as an expected 200,000 revellers begin pitching their tents.

Music lovers in France were also braving the heat, as the annual Fete de la Musique -- a nationwide celebration with thousands of free street performances -- kicked off under an official heatwave declared across many areas of the country.

The national postal service announced that its postmen would be calling on elderly and disabled residents living alone to check they were alright, with Paris sweltering in 37C heat.

In the western city of Nantes, a bevy of male bus and tram drivers made headlines with an unusual protest against a ban on their wearing shorts -- skirting the rules, literally.

"Our managers say shorts are not suitable for our profession. We've opted for provocation by coming in skirts, which are allowed for female drivers," said driver Didier Sauvetre, one of around half a dozen baring their legs.

A peak in pollution linked to the heat prompted police in the Paris region to reduce speed limits in some areas and divert heavy goods trucks, with the most polluting vehicles banned from the capital on Thursday.

On Tuesday, the body of a 31-year-old man was pulled out of the Loire river in the central city of Tours after he got dragged down by the current while trying to cool down.

In Russia, Siberia was also suffering a heatwave, with temperatures of up to 37C in the city of Krasnoyarsk, Channel One television reported.

But at the other extreme, it was just one degree Celsius and snowing in Murmansk in northern Russia, the RIA Novosti news agency reported. Many residents are having to get by with no heating in their flats as centralised systems have been switched off for the summer.